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XVI. On the Clark Cell as a Standard of Electro-motive Force. 
By Lord Rayleigh, F.R.S. 
Received Jan. 7,—Read Jan, 21, 1886. 
§ 39. The importance of a convenient standard of electro-motive force is now 
fully recognised. It gives the most available means of measuring currents, especially 
of large amount, and has been used for this purpose by several experimenters. I may 
refer to my investigation on the Constant of Magnetic Rotation of Light in Bisulphide 
of Carbon,* in which the currents were all measured by reference to a Clark cell, 
whose value was originally obtained by absolute measurements and verified at in¬ 
tervals by the silver voltameter. Clark cells are exceedingly convenient in use, and 
would doubtless be generally employed, could confidence be felt in their permanence, 
and in the equality of cells set up by different persons from the same recipe, lo 
these questions I have given much attention; and the result of a large experience is 
very favourable to the trustworthiness of the cells, if reasonable precautions be ob¬ 
served in charging them. I believe that any one who takes the trouble to set up 
three or four cells and compares them occasionally, will be in possession of a standard 
of E.M.F. which he may trust to about ToVoth part. 
The present memoir is to be regarded as supplementary to that on the Electro¬ 
chemical Equivalent of Silver, and on the Absolute Electro-motive Force of Clark 
Cells,! and the paragraphs are numbered accordingly. The total number of cells 
experimented upon is large. Of my own construction there have been about 60 of 
the ordinary kind (with solid zincs), and about 30 of the H-pattern (§ 28) with zinc 
amalgam. In addition to these some 40 cells made by others have been compared, 
with very interesting results to be given later. 
Before entering into details it may be convenient to summarise the principal 
sources of error. The E.M.F. may be too high, (1) because the paste is acid, (2) 
because the paste is not saturated with zinc sulphate. The first fault tends to cure 
itself, and is rarely found after the cells are a month old. The second is the origin of 
the more serious discrepancies that have been met with in commercial cells. If the 
E.M.F. is too low, the cause may be, (l) that the cell has become dry, in which case 
the drop will probably be progressive, (2) the solution is s?qper-saturated with zinc 
sulphate, (3) the mercury is impure. 
* See ante, p. 343. 
f Phil. Trans. Part II., 1884. 
MDCCCLXXXV. 
5 H 
